So ISO 1600 things peak on a noise.dynamic range basis. Interesting. So then to optimise the exposure I am looking at what length tracked exposure do I get good exposure at ISO1600.

My suggestion would be to choose Exposure (ss and f/number) independently of ISO: for instance in your Rho Ophiuchi 10x30s stack at f/2.8, you chose 30s and f/2.8 for good technical reasons - so stick with them. Then set ISO at base and increase it until you reach 1600 or you start clipping desirable highlights - whichever comes first. It looks like in the case above you would have set it at 1600, so the out of camera images would look a stop darker than you are used to. Not a problem, you can correct brightness to your delight in the raw converter without a SNR penalty, per my previous post. Using ISO 3200 like you did did not give you a cleaner image, but it did reduce your available DR by 1 stop unnecessarily, which looks like would have been useful.

Do you understand why the 5D3 and other Canon graph looks so wildly different in shadow DR than D800?? Does it mean for a 5D3 ISO3200 is more optimum?

Yes, if you want to maximize SNR with a 5D3 in a case such as the one above you would HAVE to go to ISO3200 at the expense of 1 stop of DR. The reason is that Canon's ADC implementation has not progressed as much as the competition over the last few years so they have less latitude and more often need to compromise SNR for DR and vice versa. Looking at their performance at DxO it appears that in a situation such as this the D800e SNR performance at ISO 1600 (or 3200/6400 - don't forget it's one and the same since it seems to be done digitally after 1600) would be better than the 5D3's at ISO 1600 and about the same as the 5D3 at ISO 3200. But with the D800e at ISO 1600 you would have 1 stop better DR than the 5D3 at 3200, and similar SNR.

Once you realize that Exposure is all about getting the most photons possible given your technical/artistic constraints, and ISO is an independent variable about processing those photons iinto a raw file while maximizing SNR and DR without clipping desirable highlights you are on your way. Give it a try and let us know if it works for you.